The present invention relates to an automobile wheel bearing unit of the kind including rolling elements, a cage, an external member to be mounted to a chassis of an automobile, an internal member to be connected with a wheel, an annular member mounted on and turning in conjunction with said internal member, and a rotational speed detector which detects a rotational speed of said annular member.
In some automobiles, a rotational velocity of each wheel is detected with a rotational speed detector and the operation of the brake cylinders is controlled according to the velocity of each wheel in order to prevent the wheels from locking when the brakes are suddenly applied on a frozen road surface.
Such a rotational speed detector as is known from the prior art is shown in FIG. 11. This wheel bearing unit comprises an inner ring 51 mounted on axle end 50 laterally projecting from a chassis as an idler, cage 52 fit over inner ring 51 and holding multiple rolling elements 53, an outer ring 55 to which is mounted wheel 54 and which is free to move around said rolling elements 53, pulser 56 which comprises a powerful magnetic body fit over said outer ring 55 and secured to ring 55 at the side thereof opposite wheel 54 and which has gear tooth-like projections extending at the circumference thereof, and speed sensor 57 which is mounted on the chassis at the base of axle end 50. A minimal gap is provided between the outer circumference of pulser 56 and speed sensor 57 which has a built-in permanent magnet and coil. When pulser 56 rotates in conjunction with wheel 54, magnetic flux emanating from the permanent magnet of speed sensor 57 is varied by the gear tooth-like projections on the outer circumference of said pulser, thus generating an AC voltage of a frequency proportional to the rotational speed of said pulser in the coil; it is therefore possible to obtain the rotational speed of wheel 54 by thus detecting the frequency of this AC voltage.
However, according to the conventional automobile wheel bearing unit, speed sensor 57 is mounted at the base of axle end 50 so as to project from protective cover 58 on the chassis. Thus, grit, mud, water and other foreign substances are able to penetrate to a detector head of speed sensor 57 as shown by arrow A through a gap between said protective cover 58 and brake shoe 59 mounted on wheel 54, thereby resulting in corrosion and damage to said detector head. Furthermore, such grit, water and other foreign substances may damage the teeth on the circumference of pulser 56, and may clog the gaps between said teeth, thus causing in either case a major problem of the loss of the ability of detect with good precision the rotational speed of the wheel.